Broadcast and multicast in single frequency networks using othrogonal space-time codes

ABSTRACT

In one embodiment, a method of transmitting includes assigning column vectors of a generalized orthogonal space time code matrix to coverage areas of a plurality of base stations such that each coverage area is assigned one column vector. The same data is transmitted from each of the plurality of base stations in the coverage areas such that the data transmitted in each coverage area is transmitted using the column vector assigned to the coverage area.

BACKGROUND OF THE INVENTION

Single Frequency Networks (SFN) are often used to support broadcastapplications where multiple users dispersed over the coverage area ofthe SFN tune to the application of common interest to all. In an SFNwith multiple base stations, signals corresponding to the broadcastapplication are transmitted in the same frequency band by all basestations. The idea is that as mobile users move from the coverage of onebase station to the next, they do not need to perform any specialactions such as handoff or tuning to a different frequency band tocontinue to receive the signals associated with the broadcastapplication. A transmission technology that appears to be well-suited toSFN-based broadcast is Orthogonal Frequency Division Multiplexing(OFDM), where base stations participating in the SFN transmit identicalsignals over the set of sub-carriers allocated to the broadcastapplication. OFDM allows (within certain limits) signals transmitted bydifferent base stations to be added at the receiver provided they alluse the same set of sub-carriers to transmit an identical set ofsignals. In a broadcast application supported by an SFN, this scheme isexpected to help receiver devices at cell edges by allowing them toprocess aggregate signals originating from multiple base stations ratherthan relying on a single base station for the received signal. However,even with OFDM, destructive interaction can take place between signalsoriginating from different base stations because of the relative phasedifferences.

SUMMARY OF THE INVENTION

The present invention relates to a method of transmitting data in asingle frequency network.

In one embodiment, the method includes assigning column vectors of ageneralized orthogonal space time code matrix to coverage areas of aplurality of base stations such that each coverage area is assigned onecolumn vector. The same data is transmitted from each of the pluralityof base stations in the coverage areas such that the data transmitted ineach coverage area is transmitted using the column vector assigned tothe coverage area.

The present invention also relates to a single frequency network.

In one embodiment, the single frequency network includes a plurality ofbase stations. The base stations are configured to perform at least oneof broadcasting and multicasting data over a same frequency. Theplurality of base stations each have at least one coverage area, andeach coverage area is assigned a column vector of a generalizedorthogonal space time code matrix. The plurality of base stationstransmit the broadcast or multicast data in each coverage area using thecolumn vector assigned to the coverage area.

The present invention still further relates to a method of receivingdata in a single frequency.

In one embodiment, the method includes deriving a modified receivedsignal vector from received signal samples, and multiplying the modifiedreceived signal vector by a receiver filter matrix. The receiver filtermatrix includes at least one channel estimate coefficient associatedwith each column vector of a generalized orthogonal space time code toobtain decision metrics. Data represented by the received signal samplesis decoded based on the obtained decision metrics.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawings,wherein like elements are represented by like reference numerals, whichare given by way of illustration only and thus are not limiting of thepresent invention and wherein:

FIG. 1 shows a cellular arrangement of the base stations participatingin an SFN where each base station has an omni-directional antenna and,therefore, a single coverage area.

FIG. 2 illustrates such an example where each base station has a3-sector antenna and therefore three coverage areas.

FIG. 3 shows another cellular arrangement of the base stationsparticipating in an SFN.

FIG. 4 illustrates an assignment of the first block of 4K modulated datasymbols to K sub-carriers.

FIGS. 5A, 5B and 5C show the symbols that are actually transmitted onthe K sub-carriers by a base station transmitter that has been assignedcolumn vectors A, B and C, respectively, of the code matrix.

FIG. 6 shows a possible assignment of data symbols to sub-carrier groupsaccording to one embodiment.

FIG. 7 shows a possible assignment of the eight transmitted symbols tothe eight transmission opportunities provided by the kth sub-carriergroup at a base station that has been assigned column A of the codematrix.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Illustrative embodiments are described below. In the interest ofclarity, not all features of an actual implementation are described inthis specification. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions should be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

In the following description, for purposes of explanation and notlimitation, specific details are set forth such as particulararchitectures, interfaces, techniques, etc., in order to provide athorough understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention may bepracticed in other illustrative embodiments that depart from thesespecific details. In some instances, detailed descriptions of well-knowndevices, circuits, and methods are omitted so as not to obscure thedescription of the present invention with unnecessary detail. Allprinciples, aspects, and embodiments of the present invention, as wellas specific examples thereof, are intended to encompass both structuraland functional equivalents thereof. Additionally, it is intended thatsuch equivalents include both currently known equivalents as well asequivalents developed in the future.

Exemplary embodiments are discussed herein as being implemented in asuitable computing environment. Although not required, exemplaryembodiments will be described in the general context ofcomputer-executable instructions, such as program modules or functionalprocesses, being executed by one or more computer processors or CPUs.Generally, program modules or functional processes include routines,programs, objects, components, data structures, etc. that performparticular tasks or implement particular abstract data types. Theprogram modules and functional processes discussed herein may beimplemented using existing hardware in existing communication networks.For example, program modules and functional processes discussed hereinmay be implemented using existing hardware at existing network elementsor control nodes. Such existing hardware may include one or more digitalsignal processors (DSPs), application-specific-integrated-circuits,field programmable gate arrays (FPGAs) computers or the like.

Portions of the embodiments and corresponding detailed description arepresented in terms of software, or algorithms and symbolicrepresentations of operations on data bits within a computer memory.These descriptions and representations are the ones by which those ofordinary skill in the art effectively convey the substance of their workto others of ordinary skill in the art. An algorithm, as the term isused here, and as it is used generally, is conceived to be aself-consistent sequence of steps leading to a desired result. The stepsare those requiring physical manipulations of physical quantities.Usually, though not necessarily, these quantities take the form ofoptical, electrical, or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise, or as is apparent from the discussion,terms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical, electronicquantities within the computer system's registers and memories intoother data similarly represented as physical quantities within thecomputer system memories or registers or other such information storage,transmission or display devices.

Note also that the software implemented aspects of the invention aretypically encoded on some form of program storage medium or implementedover some type of transmission medium. The program storage medium may bemagnetic (e.g., a floppy disk or a hard drive) or optical (e.g., acompact disk read only memory, or “CD ROM”), and may be read only orrandom access. Similarly, the transmission medium may be twisted wirepairs, coaxial cable, optical fiber, or some other suitable transmissionmedium known to the art. The embodiments are not limited by theseaspects of any given implementation.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments of thepresent invention. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between”, “adjacent” versus “directlyadjacent”, etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments of the invention. As used herein, the singular forms “a”,“an” and “the” are intended to include the plural forms as well, unlessthe context clearly indicates otherwise. It will be further understoodthat the terms “comprises”, “comprising,”, “includes” and/or“including”, when used herein, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

The embodiments will now be described with reference to the attachedfigures. Various structures, systems and devices are schematicallydepicted in the drawings for purposes of explanation only and so as tonot obscure the present invention with details that are well known tothose skilled in the art. Nevertheless, the attached drawings areincluded to describe and explain illustrative examples. Whereapplicable, the words and phrases used herein should be understood andinterpreted to have a meaning consistent with the understanding of thosewords and phrases by those skilled in the relevant art.

As used herein, the term “mobile station” may be considered synonymousto, and may hereafter be occasionally referred to, as a client, mobile,mobile unit, mobile station, mobile user, user equipment (UE),subscriber, user, remote station, access terminal, receiver, etc., andmay describe a remote user of wireless resources in a wirelesscommunication network. The wireless resource may be a mobile phone,wireless equipped personal computer, etc. The term “base station” may beconsidered synonymous to and/or referred to as a base transceiverstation (BTS), NodeB, extended Node B, femto cell, access point, etc.and may describe equipment that provides the radio-frequency (RF) andbaseband functions for data and/or voice connectivity between a networkand one or more users.

As is well-known in the art, each of a mobile and a base station mayhave transmission and reception capabilities. Transmission from the basestation to the mobile is referred to as downlink or forward linkcommunication. Transmission from the mobile to the base station isreferred to as uplink or reverse link communication.

Consider a Single Frequency Network (SFN) supporting a broadcastapplication. For illustrative purposes, we assume that the SFN uses amulti-carrier transmission scheme such as Orthogonal Frequency DivisionMultiplexing (OFDM). In such a scheme, identical signals are transmittedby each of the participating base stations on each tone or sub-carrierbeing used for the broadcast application. Moreover, these signals aretime-aligned within permissible limits. Now, if a receiver devicelistening to the broadcast application receives signals from multiplebase stations, the difference between the transmission delayscorresponding to different base stations would cause the signals toarrive at the receiver at somewhat different times. However, as long asthe relative delays for different base stations are within a certainlimit (corresponding to the cyclic prefix in an OFDM system), there isno inter-symbol interference due to this delay spread, which in manyother transmission technologies can only be mitigated with sophisticatedequalization techniques.

Now consider a receiver device (e.g., a mobile station) that receivessignals from N base stations participating in an SFN using an OFDMtransmission scheme. Let x^((k))(t) denote the symbol transmitted by allof these base stations using the k^(th) sub-carrier during time-slot t.The corresponding received signal is then given by

$\begin{matrix}{{{r^{(k)}(t)} = {{\sum\limits_{i = 1}^{N}{{h_{i}^{(k)}(t)}{x^{(k)}(t)}}} + {n^{(k)}(t)}}},} & (1)\end{matrix}$where for i=1, 2, . . . , N, h_(i) ^((k))(t) denotes the channelcoefficient for the signal transmitted by the i^(th) base station overthe k^(th) sub-carrier during time-slot t, and n^((k))(t) represents thethermal noise in the corresponding received signal. Note that as theabove equation indicates, the signals being received from different basestations cannot be separated so that the entire received signal for anysub-carrier (say, k) appears as if it is being received over a channelwith channel coefficient given by

$\begin{matrix}{{h^{(k)}(t)} = {\sum\limits_{i = 1}^{N}{{h_{i}^{(k)}(t)}.}}} & (2)\end{matrix}$The resulting signal-to-interference-plus-noise ratio (SINR), denoted byρ^((k))(t), equals

$\begin{matrix}{{{\rho^{(k)}(t)} = {{{\sum\limits_{i = 1}^{N}{h_{i}^{(k)}(t)}}}^{2}{{E\left\lbrack {{x^{(k)}(t)}}^{2} \right\rbrack}/\sigma^{2}}}},} & (3)\end{matrix}$where σ² represents the variance of the sum of receiver noise andinterference. Note that SFN, transmissions from neighboring basestations generally improve the signal rather than introduceinterference. The exception is the distant base stations whosetransmissions are received outside the cyclic prefix of the OFDM system.These (latter) transmissions, which contribute interference to thereceived signal, are often negligible due to distance loss. Now, the Nchannel coefficients, h_(i) ^((k))(t), are uncorrelated in phase sincethey are associated with different base stations. As a consequence, theaggregate channel coefficient, h^((k))(t), can have a large or smallmagnitude depending on whether the individual channel coefficients addconstructively or destructively. Thus, the signal quality for a givenreceiver can be extremely poor even if the aggregate signal power, i.e.the sum of the individual signal powers, received from base stationswithin its range is acceptable. In a broadcast or multicast applicationrequiring a given data rate, often the objective is to serve at least acertain fraction (e.g. 95%) of the potential user population in asefficient a manner as possible. Whether this objective can be met at agiven transmit power level is determined by the lower percentiles (e.g.5^(th) percentile if at least 95% of the population is to be served) ofthe SINR distribution. Destructive signal addition caused by phasedifferences suppresses the lower percentiles of the SINR distribution,which means that a higher transmit power needs to be used in order tomeet the coverage objective. Depending on the operational and deviceconstraints, higher transmit powers may not always be feasible.

Since we have assumed an OFDM or a similar multi-carrier system, whichallows us to consider each sub-carrier independently, we drop the indexindicating the sub-carrier of interest from our equations. Similarly,with the further assumption that our equations address the system at aparticular time instant, we drop the time parameter, t, from ourequations as well. Thus, we re-write equations (1), (2) and (3) as

$\begin{matrix}{{r = {{\sum\limits_{i = 1}^{N}{h_{i}x}} + n}},} & \left( {1a} \right) \\{{h = {\sum\limits_{i = 1}^{N}h_{i}}},{and}} & \left( {2a} \right) \\{{\rho = {{{\sum\limits_{i = 1}^{N}h_{i}}}^{2}{{E\left\lbrack {x}^{2} \right\rbrack}/\sigma^{2}}}},} & \left( {3a} \right)\end{matrix}$with the implicit assumption that we are talking about a specificsub-carrier during a particular time-slot.

A communication system employing generalized complex orthogonalspace-time codes is one where complex data symbols (e.g. thoseassociated with QPSK or M-QAM signaling) to be transmitted are firstgrouped in sets of m symbols, and then each such set of m symbols istransmitted from n antennas over p time-slots in the following manner:Over the p time-slots, each of the n antennas gets to transmit each ofthe m symbols (possibly modified via “negation” and/or “conjugation”);moreover, the vectors of symbols transmitted over the p time-slots fromany pair of antennas are mutually orthogonal. Since m symbols aretransmitted over p time-slots, the coding rate for this transmissionscheme works out to m/p. The p-dimensional symbol vectors associatedwith each of the n antennas can be arranged as columns of a p×n matrix,G, which is referred to as the code matrix. The following example is anillustration of a generalized complex space-time code.

In this example embodiment, complex data symbols are grouped into setsof four symbols each, and then each set of 4 symbols (i.e., m=4) aretransmitted from 3 antennas (n=3) over 8 time-slots (p=8). Thecorresponding G matrix is as shown below, where the four data symbolsare denoted by x₁, x₂, x₃ and x₄ and, for i=1, 2, 3 and 4, x_(i)*denotes the complex conjugate of x_(i).

$\begin{matrix}{G = \begin{bmatrix}x_{1} & x_{2} & x_{3} \\{- x_{2}} & x_{1} & {- x_{4}} \\{- x_{3}} & x_{4} & x_{1} \\{- x_{4}} & {- x_{3}} & x_{2} \\x_{1}^{*} & x_{2}^{*} & x_{3}^{*} \\{- x_{2}^{*}} & x_{1}^{*} & {- x_{4}^{*}} \\{- x_{3}^{*}} & x_{4}^{*} & x_{1}^{*} \\{- x_{4}^{*}} & {- x_{3}^{*}} & x_{2}^{*}\end{bmatrix}} & (4)\end{matrix}$

In this example embodiment, the four data symbols are transmitted over 8time-slots in the following manner: the first antenna transmits thesymbols indicated by the first column of the G matrix (i.e., the firstantenna transmits the symbols x₁, −x₂, −x₃, −x₄, x₁*, −x₂*, −x₃* and−x₄* over the 8 time-slots); the second antenna transmits the symbolsindicated by the second column of G; and the third antenna transmitsthose indicated by the third column of G. It is easy to verify that anytwo columns of the code matrix G are mutually orthogonal. The rateassociated with the corresponding generalized orthogonal space-time codeis 4/8=½. It has been shown that a rate ½ generalized complex orthogonalcode exists for any positive integer value of the parameter n, whichcorresponds to the number of transmit antennas.

Now consider a receiver device (e.g., a mobile station) that receivessignals transmitted by all three antennas in the current example. Leth₁, h₂, h₃ respectively denote the channel coefficients between thethree transmit antennas and the receiver device. We assume that thethree channel coefficients remain constant for the eight time-slots overwhich the four data symbols belonging to one symbol set are transmitted.With this assumption, we can writer=Gh+n,  (5)where the 8-dimensional vector r(=[r₁, r₂, . . . , r₈]^(T)) representsthe received signal over the eight time-slots, and entries of the8-dimensional vector n(=[n₁, n₂, . . . , n₈]^(T)) represent thecorresponding noise components. The 3-dimensional vector h equals [h₁ h₂h₃]^(T). (The notation (.)^(T) represents the transpose of thecorresponding matrix or vector.)

Now, let ŕ=[ŕ₁, ŕ₂, . . . , ŕ₈]^(T)=[r₁, r₂, r₃, r₄, r₅*, r₆*, r₇*,r₈*]^(T). We refer to the vector {acute over (r)} as the modifiedreceived signal vector. It is easy to verify that we can write{acute over (r)}=Ĥx,+ń,  (6)where x equals [x₁, x₂, x₃, x₄]^(T), {acute over (n)} equals [n₁, n₂,n₃, n₄, n₅*, n₆*, n₇*, n₈*]^(T), and the matrix Ĥ is given by:

$\begin{matrix}{\hat{H} = \begin{bmatrix}{h\; 1} & {h\; 2} & {h\; 3} & 0 \\{h\; 2} & {{- h}\; 1} & 0 & {{- h}\; 3} \\{h\; 3} & 0 & {{- h}\; 1} & {h\; 2} \\0 & {h\; 3} & {{- h}\; 2} & {{- h}\; 1} \\{h\; 1^{*}} & {h\; 2^{*}} & {h\; 3^{*}} & 0 \\{h\; 2^{*}} & {{- h}\; 1^{*}} & 0 & {{- h}\; 3^{*}} \\{h\; 3^{*}} & 0 & {{- h}\; 1^{*}} & {h\; 2^{*}} \\0 & {h\; 3^{*}} & {{- h}\; 2^{*}} & {{- h}\; 1^{*}}\end{bmatrix}} & (7)\end{matrix}$Note that columns of the matrix Ĥ are mutually orthogonal and that thesquare of the magnitude of each column equals 2(|h₁|²+|h₂|²+|h₃|²).Therefore, if we pre-multiply (6) by Ĥ*^(T), we obtain

$\begin{matrix}{{\hat{\underset{\_}{w}} = {\begin{bmatrix}{\hat{W}}_{1} \\{\hat{W}}_{2} \\{\hat{W}}_{3} \\{\hat{W}}_{4}\end{bmatrix} = {{{\hat{H}}^{*T}\overset{\prime}{\underset{\_}{r}}} = {{2\left( {{h_{1}}^{2} + {h_{2}}^{2} + {h_{3}}^{2}} \right)\underset{\_}{x}} + \overset{\Cup}{\underset{\_}{n}}}}}},} & (8)\end{matrix}$where {circumflex over (n)}=Ĥ*^(T){acute over (n)}. This means thatafter the simple linear processing of the vector {acute over (r)},represented by the pre-multiplication by Ĥ*^(T), the components of theoutput vector ŵ equal a simple sum of the corresponding data symbol(with some amplification) and a noise component. We refer to the matrixĤ*^(T) as the receiver filter matrix. For a given set of channelcoefficients, h₁, h₂, and h₃, the expected value of the square of themagnitude of the signal component corresponding to any data symbol x_(i)is 4(|h|²+|h₂|²+|h₃|²)²E[|x_(i)|²] whereas the variance of the noisecomponent equals 2(|h₁|²+|h₂|²+|h₃|²)σ² where σ²=E[|n_(i)|²] in (5).This results in a signal-to-noise ratioρ_(i)=2(|h ₁|² +|h ₂|² +|h ₃|²)E[|x _(i)|²]/σ².  (9)

The expression for the SINR at a receiver where signals transmitted fromsome of the antennas are too weak for reception can be obtained bysetting the corresponding channel coefficients to 0 in equation (8).Note that for a receiver to be able to decode the transmitted symbols,it is not necessary to receive signals transmitted from all threeantennas. As long as the SINR resulting from the signals received fromone or more antennas is strong enough, the receiver can successfullydecode the transmitted symbols. While we have used an example of aspecific generalized orthogonal space-time code (with m=4, n=3 and p=8),all such space-time codes exhibit the properties described above.

It is revealing to compare equation (9) with equation (3). The factor“2” that appears in the expression for SINR in (9) can be attributed tothe fact that each antenna effectively transmits every data symbol twicein a system employing the orthogonal space-time code described above.However, the key difference is the other factor—“(|h₁|²+|h₂|²+|h₃; |²)”in the case of the system using an orthogonal space-time code with n=3,and “(|h₁+h₂+h₃|²)” for an ordinary SFN with n=3 transmit antennas. Aswe mentioned earlier, because of the relative phases associated with thechannel coefficients corresponding to different transmit antennas, thequantity |h₁+h₂+h₃|² can be unacceptably small even when the sum ofindividual signal powers received from different antennas is adequate.On the other hand, the quantity “|h₁|²+|h₂|²+|h₃|²” corresponds to thesum of the individual signal powers, and, as such, does not suffer frompossibly destructive addition of signal components because of phasedifferences. In fact, signals being received from different antennasmake positive contributions to the overall signal quality, affording afull diversity benefit to the receiver. Consequently, the lowerpercentiles of the SINR distribution, which, as we saw earlier,determine the transmit power needed to provide the desired coverage,exhibit marked improvement when compared to ordinary SFN networks.

The above discussion assumes that there is a single antenna at thereceiver. It is easy to see how the analysis can be extended toreceivers with multiple antennas. Specifically, for a receiver with Kantennas, the receiver filter matrix, Ĥ*^(T), is determined for each ofthe K antennas. Then, independently for each antenna, the receiverobtains the decision metric vector ({circumflex over (w)}) bypre-multiplying the corresponding modified received signal vector withthe receiver filter matrix associated with that antenna. The finaldecision metrics are then computed by summing the decision metricvectors corresponding to all receiver antennas.

The preceding analysis shows that if a set of antennas transmitted acommon stream of data symbols using different (mutually orthogonal)column vectors of a generalized orthogonal space time code, a receiverdevice that is within the range of a subset of these antennas can decodethe (common) data stream without suffering from the potentiallydestructive interaction between signals caused by relative phasedifferences. Thus, one way to achieve such an arrangement is toconstruct a generalized orthogonal space-time code with parameter n(which corresponds to the number of column vectors of the code matrix G)large enough so that each base station antenna participating in the SFNcan be assigned a distinct column vector. This would ensure that nomatter where the receiver device is located, if it receives signals frommultiple antennas participating in the SFN, those antennas will betransmitting the common data stream using mutually orthogonal columnvectors of the code matrix G. As a consequence, there will be nodestructive interaction between the signals coming from differentantennas because of relative phase differences.

However, in large SFNs, each with possibly hundreds of base stations, itis impractical to adopt a generalized orthogonal space-time code where adistinct orthogonal column vector can be assigned to each base stationantenna participating in the SFN. This is due to the following reason:although generalized orthogonal codes (at rate ½) exist for any value ofthe parameter n (which corresponds to the number of distinct columnvectors of the code matrix), a large value of this parameter forces theother parameter of the code (i.e. p, which represents the number of rowsof the code matrix) to be large as well. Since the efficacy of the code(which depends on the orthogonality of the column vectors of the Ĥmatrix) improves with less channel variation over the p time-slots usedby each antenna to transmit its column vector, a large value of theparameter p may have a detrimental effect on performance or limitapplicability, especially in today's world where a large fraction ofusers are likely to be mobile and have time-varying channels. What thismeans is that the parameters n and p of the space-time code will have tobe reasonably small if the code is to be effective in an SFN with mobileusers. We address this apparently conflicting set of requirementsthrough code (vector) reuse.

Note that from a receiver's perspective, it does not matter whether allthe base station antennas participating in the SFN use distinct(orthogonal) column vectors of the space-time code. As long as theantennas whose signals are strong enough when they reach the receiverdevice use distinct column vectors of the space-time code, those signalswill not combine in a destructive manner due to phase differences. Thus,we use a (relatively) small space-time code and assign the columnvectors of its code matrix to different base station antennas in such amanner that at points in the coverage area of the SFN where signals frommultiple base station antennas are received, the column vectors assignedto those base station antennas are likely to be distinct. We try toensure that if the signals from any given set of base station antennascan reach some points at significant levels, those base station antennasare assigned distinct column vectors of the space-time code. Weillustrate this concept with a few examples. In the following, we referto the number of column vectors of the code matrix G (i.e. the parametern of the space-time code) as the “reuse parameter.”

Consider the generalized orthogonal space-time code represented by thecode matrix G in equation (4). We refer to the first, second and thirdcolumn vectors of G respectively as columns A, B and C, respectively.FIG. 1 shows a cellular arrangement of the base stations participatingin an SFN. It is assumed here that all base stations haveomni-directional antennas, which would result in a hexagonalcell-pattern under ideal conditions. Thus each base station has a singlecoverage area. The base stations (and the corresponding cells) arenumbered 1, 2, 3, . . . , and the locations of the base station antennasare marked with an “x.” A letter “A,” “B” or “C” appears next to eachnumber representing the identifier of the corresponding base station.This letter refers to the column vector of the code matrix G that hasbeen assigned to the corresponding base station antenna and/or coveragearea. Thus, the base station 1's antenna and/or coverage area has beenassigned the column vector A, base station 2's antenna and/or coveragearea has been assigned the column vector C, and so on. As shown in FIG.1, the coverage areas are assigned the A, B, and C column vectors insuch a manner that no two adjacent coverage areas have the same columnvector assignment.

In the case of a broadcast application, each base station participatingin the SFN continuously transmits signals associated with thatapplication in accordance with the column vector of the code matrixassigned to it. This happens regardless of whether any receiver deviceis within range of that base station.

The operation of a multicast application, in accordance with the presentinvention, is a little different. In an SFN supporting a multicastapplication, each base station is permanently assigned a column vectorof the code matrix (just as in the case of a broadcast application.)However, a base station transmits signals associated with the multicastapplication only if one or more user devices listening to thatapplication are within its range. Thus, normally, when there are no userdevices within range of a base station, the latter does not transmit anysignals associated with the multicast operation. When a user devicewithin its range tunes to the multicast or a device already tuned to themulticast moves within its range (e.g., notified by uplink signaling),the base station starts transmitting signals associated with themulticast. When the base station transmits these signals, the basestation encodes them using the column vector of the code matrix assignedto the base station (just as in a broadcast application.) Note that evenin an SFN supporting a multicast application, all base stations useidentical sub-carriers and are synchronized with one another withinpermissible limits. We would also like to point out that a user devicecan be within range of several base stations at the same time, e.g. whenit is in a “soft handoff” area.

Now consider a mobile station y in the coverage area of cell 1 as shownin FIG. 1. Since mobile station y is close to that cell's boundary withcells 6 and 7, the mobile station y is likely to receive signals fromthe antennas of base stations 6 and 7 in addition to those from basestation I's antenna. However, as one can see in FIG. 1, the threeantennas use three distinct column vectors of the code matrix G. As aconsequence, their signals will not interfere destructively with oneanother when the receiver in mobile station y attempts to decode them.Namely, the mobile station y will receive and decode the signals asexemplified by equation (8). It will be appreciated that channelestimation techniques are well-known, and any such technique may be usedto derive the matrix h, and thus Ĥ or Ĥ*^(T). Similarly, a mobilestation z in cell 2's coverage area receives signals from the antennasof base station 2 and base station 9. Once again, we can see that theseantennas use distinct column vectors of matrix G so that there is nodestructive interference between the signals transmitted by theseantennas when the receiver of mobile station z attempts to decode them.

FIG. 1 illustrates the concept of code (vector) reuse in an SFN withomni-directional base station antennas. Similar code (vector) reuse canbe implemented in SFNs with sectorized antennas as well. FIG. 2illustrates such an example where each base station has a 3-sectorantenna and therefore three coverage areas. Only the column vectorsassigned to different sectors have been shown in FIG. 2; the basestation and sector identifiers have been omitted to avoid cluttering ofthe figure. The same space-time code with reuse parameter 3 that wasused in the previous example is used here as well. With sectorized basestation antennas, each sector (i.e., coverage area) of a base stationneeds to be assigned a different column vector. It is easy to see thatthe column vector assignment shown in FIG. 2 would ensure that if amobile station receives signals from multiple sectors (of the same cellor different cells), those sectors are likely to be using differentcolumn vectors of the code matrix. As shown in FIG. 2, each upper rightsector of a base station is assigned column vector A, each bottom sectorof a base station is assigned column vector B, and each upper leftsector is assigned column vector C. This embodiment is not limited tothese column vector assignments. Instead, the column vector assignmentsfor each base station should be the same, which insures that no twoadjacent sectors have a same column vector assignment.

We have used a generalized orthogonal space-time code with a 3-columncode matrix to illustrate the concepts of the example embodiments. It iseasy to see how generalized orthogonal space-time codes with codematrices of different sizes can be used to implement different reusepatterns. For instance, for a reuse pattern with 7 distinct columnvectors, one could use the generalized orthogonal space-time code withcode matrix G₂ shown below. The resulting column vector assignment isshown in FIG. 3, where the first through seventh columns of G₂ have beenrespectively labeled A, B, C, D, E, F and G.

$\begin{matrix}{G_{2} = \begin{bmatrix}{x\; 1} & {x\; 2} & {x\; 3} & {x\; 4} & {x\; 5} & {x\; 6} & {x\; 7} \\{{- x}\; 2} & {x\; 1} & {x\; 4} & {{- x}\; 3} & {x\; 6} & {{- x}\; 5} & {{- x}\; 8} \\{{- x}\; 3} & {{- x}\; 4} & {x\; 1} & {x\; 2} & {x\; 7} & {x\; 8} & {{- x}\; 5} \\{{- x}\; 4} & {x\; 3} & {{- x}\; 2} & {x\; 1} & {x\; 8} & {{- x}\; 7} & {x\; 6} \\{{- x}\; 5} & {{- x}\; 6} & {{- x}\; 7} & {{- x}\; 8} & {x\; 1} & {x\; 2} & {x\; 3} \\{{- x}\; 6} & {x\; 5} & {{- x}\; 8} & {x\; 7} & {{- x}\; 2} & {x\; 1} & {{- x}\; 4} \\{{- x}\; 7} & {x\; 8} & {x\; 5} & {{- x}\; 6} & {{- x}\; 3} & {x\; 4} & {x\; 1} \\{{- x}\; 8} & {{- x}\; 7} & {x\; 6} & {x\; 5} & {{- x}\; 4} & {{- x}\; 3} & {x\; 2} \\{x\; 1^{*}} & {x\; 2^{*}} & {x\; 3^{*}} & {x\; 4^{*}} & {x\; 5^{*}} & {x\; 6^{*}} & {x\; 7^{*}} \\{{- x}\; 2^{*}} & {x\; 1^{*}} & {x\; 4^{*}} & {{- x}\; 3^{*}} & {x\; 6^{*}} & {{- x}\; 5^{*}} & {{- x}\; 8^{*}} \\{{- x}\; 3^{*}} & {{- x}\; 4^{*}} & {x\; 1^{*}} & {x\; 2^{*}} & {x\; 7^{*}} & {x\; 8^{*}} & {{- x}\; 5^{*}} \\{{- x}\; 4^{*}} & {x\; 3^{*}} & {{- x}\; 2^{*}} & {x\; 1^{*}} & {x\; 8^{*}} & {{- x}\; 7^{*}} & {x\; 6^{*}} \\{{- x}\; 5^{*}} & {{- x}\; 6^{*}} & {{- x}\; 7^{*}} & {{- x}\; 8^{*}} & {x\; 1^{*}} & {x\; 2^{*}} & {x\; 3^{*}} \\{{- x}\; 6^{*}} & {x\; 5^{*}} & {{- x}\; 8^{*}} & {x\; 7^{*}} & {{- x}\; 2^{*}} & {x\; 1^{*}} & {{- x}\; 4^{*}} \\{{- x}\; 7^{*}} & {x\; 8^{*}} & {x\; 5^{*}} & {{- x}\; 6^{*}} & {{- x}\; 3^{*}} & {x\; 4^{*}} & {x\; 1^{*}} \\{{- x}\; 8^{*}} & {{- x}\; 7^{*}} & {x\; 6^{*}} & {x\; 5^{*}} & {{- x}\; 4^{*}} & {{- x}\; 3^{*}} & {\;{x\; 2^{*}}}\end{bmatrix}} & (10)\end{matrix}$

System Implementation

As we saw earlier, in an SFN employing the OFDM technology, in order tosupport a broadcast application all antennas participating in the SFNhave to use the same set of sub-carriers to carry the broadcast datastream. Moreover, if an antenna uses a specific sub-carrier to transmita given set of data symbols within the broadcast stream, all antennas inthe SFN must use the same sub-carrier to transmit that set of datasymbols. Also, each data symbol has to be modulated in an identicalmanner at all antennas. Changes to this mode of operation are made whenwe employ generalized orthogonal space-time coding with code (vector)reuse in an SFN according to the above described embodiments. Wedescribe these changes using the space-time code with reuse factor 3 asan illustrative example.

Let us assume that K sub-carriers have been assigned at each basestation to support the broadcast application. Although thesesub-carriers can be anywhere in the spectrum being used by the SFN, werefer to these sub-carriers by the indices 1, 2, . . . , K. Themodulated data symbols in the broadcast stream (which could be in a QPSKor M-QAM format) are denoted by z1, z2, z3, . . . . Recall that with thespace-time code with reuse factor 3, four data symbols are transmittedover eight time-slots on each sub-carrier. Therefore, one way toimplement this in a systematic manner would be to have each base station(or sector in the case of sectorized antennas) transmitter group themodulated data symbols in sets of size 4K, where K is the number ofsub-carriers. Each such set of data symbols will be referred to as a“block.” Thus, the first such block would be: z1, z2, z3, . . . , z4K.The second such block would be: z4K+1, z4K+2, . . . , z8K, and so on.When the first block of 4K data symbols is ready for transmission, thetransmitter assigns the symbols z1, zK+1, z2K+1, and z3K+1 to the firstsub-carrier, the symbols z2, zK+2, z2K+2, and z3K+2 to the secondsub-carrier, and so on. A similar assignment is carried out when eachblock of 4K modulated data symbols is ready for transmission. FIG. 4illustrates this assignment for the first block of 4K modulated datasymbols.

In accordance with the embodiments, the assignment of modulated datasymbols to sub-carriers is performed in an identical manner (as shown inFIG. 4) by all base station transmitters. However, the symbols that areactually transmitted over the next 8 time-slots by a base stationtransmitter depend on the column vector of the code matrix G assigned tothe base station. For instance, consider a base station that has beenassigned column vector A (i.e. the first column) of the code matrix G.Equation (4) shows that if modulated data symbols x1, x2, x3 and x4 areto be transmitted on a particular sub-carrier over eight time-slots, thesymbols that are actually transmitted by that base station over thisperiod are: x1, −x2, −x3, −x4, x1, −x2*, −x3* and −x4*. The base stationindependently applies this transformation to the set of symbols assignedto each sub-carrier to obtain the corresponding set of symbols that areactually transmitted over the corresponding eight time-slots. FIG. 5Ashows the symbols that are actually transmitted on the K sub-carriers bya base station transmitter that has been assigned column vector A of thecode matrix.

For the same set of modulated data symbols (i.e. z1, z2, z3, . . . ,z4K), FIGS. 5B and 5C show the symbols that are actually transmitted bybase stations that have been assigned column vectors B and C,respectively.

At the receiver device (e.g., mobile station), the signals received overthe eight time-slots used to transmit one block of data symbols aredemodulated in the following manner:

For each sub-carrier k, where k=1, 2, . . . , K, the receiver of themobile station maintains three estimates of channel coefficients:ĥ_(A)(k), ĥ_(B)(k) and ĥ_(C)(k), where ĥ_(A)(k) is the receiver'sestimate of the aggregate channel coefficient (for sub-carrier k) forall base stations that have been assigned the column vector A. As we sawpreviously, signals from two base stations can be separated (because ofthe space-time code) only if they use different column vectors of thecode matrix. Conversely, if a receiver receives signals from multiplebase stations using the same column vectors of the code matrix, thereceiver cannot separate the signals into components that can beattributed to different base stations. As a consequence, as far assignals from base stations using the same column vector are concerned,the receiver can only deal with sums (i.e. aggregates) of these signals.The channel coefficients in this case are also aggregates; that is, theyare the receiver's estimate of the “sum” channel, in which the signalsfrom all base stations with the same column vector are added togetherbefore the receiver processes them. Similarly, ĥ_(B)(k) and ĥ_(C)(k) arethe receiver's aggregate channel estimates for all base stations thathave been assigned column vectors B and C, respectively. These estimatescan be obtained via standard techniques such as periodically insertedpilot symbols, etc. Any of these aggregate channel coefficients can be 0if the signals from the corresponding base stations are too weak to bedetected by the receiver device. For each sub-carrier k (where k=1, 2, .. . , K), the receiver forms the 8×3 matrix Ĥ(k) as follows:

$\begin{matrix}{{\hat{H}(k)} = \begin{bmatrix}{{\hat{h}}_{A}(k)}^{*} & {{\hat{h}}_{B}(k)}^{*} & {{\hat{h}}_{C}(k)}^{*} & 0 \\{{\hat{h}}_{B}(k)}^{*} & {- {{\hat{h}}_{A}(k)}^{*}} & 0 & {- {{\hat{h}}_{C}(k)}^{*}} \\{{\hat{h}}_{C}(k)}^{*} & 0 & {- {{\hat{h}}_{A}(k)}^{*}} & {{\hat{h}}_{B}(k)}^{*} \\0 & {{\hat{h}}_{C}(k)}^{*} & {- {{\hat{h}}_{B}(k)}^{*}} & {- {{\hat{h}}_{A}(k)}^{*}} \\{{\hat{h}}_{A}(k)}^{*} & {{\hat{h}}_{B}(k)}^{*} & {{\hat{h}}_{C}(k)}^{*} & 0 \\{{\hat{h}}_{B}(k)}^{*} & {- {{\hat{h}}_{A}(k)}^{*}} & 0 & {- {{\hat{h}}_{C}(k)}^{*}} \\{{\hat{h}}_{C}(k)}^{*} & 0 & {- {{\hat{h}}_{A}(k)}^{*}} & {{\hat{h}}_{B}(k)}^{*} \\0 & {{\hat{h}}_{C}(k)}^{*} & {- {{\hat{h}}_{B}(k)}^{*}} & {- {{\hat{h}}_{A}(k)}^{*}}\end{bmatrix}} & (11)\end{matrix}$

For each sub-carrier k, (k=1, 2, . . . , K), the receiver collectsreceived signal samples for one block of transmitted symbols. Thus,there are eight received signal samples for each sub-carrier k. Let r_(k) denote the column vector of these eight received signal samples forsub-carrier k. From the vector r _(k), the receiver obtains the8-dimensional column vector ŕ _(k), whose first four elements areidentical to the first four elements of r _(k), and the other fourelements are complex conjugates of the corresponding elements of r _(k).The vector ŕ _(k) is then pre-multiplied by Ĥ(k)*^(T) to obtain the4-dimensional vector {circumflex over (w)}_(k), whose elements form thedecision metrics corresponding to the four modulated data symbolstransmitted over the sub-carrier k. Similar processing is carried outfor all sub-carriers k (k=1, 2, . . . , K) to obtain decision metricsfor all of the modulated data symbols transmitted in a block. Thesedecision metrics are used in standard decoding procedures to obtain anestimate of the original data.

Note that in the above example as well as in the earlier description itwas assumed that a set of modulated data symbols was assigned to asub-carrier and that the corresponding transmitted symbols were sentusing that sub-carrier over “p” consecutive time-slots allocated to ablock of data symbols. This arrangement has two implications—it entailsan average processing delay of p/2 time-slots (since the entire block ofdata needs to be available at the receiver for processing); also, itrequires the channel coefficients to be (more-or-less) constant over ptime-slots. This may not be a major concern in several applications.However, in those cases where either the processing delay of ptime-slots is unacceptable or where the channel cannot be expected toremain constant for p time-slots (say, because of the mobility ofreceiver devices), it is possible to reduce the delay by a significantamount. We accomplish this by exploiting the fact that in manysituations the channel coefficients associated with adjoiningsub-carriers are close to each other. As a result, we can group thesub-carriers into groups, each consisting of L contiguous sub-carriers,and assign each set of m modulated symbols to one such sub-carriergroup. Since these m symbols require p transmission opportunities, wecan transmit them over p/L time-slots since the sub-carrier grouppresents us with L transmission opportunities during each time-slot.This reduces the processing delay by a factor of L. We illustrate thisconcept with the following example where the same space-time code (withreuse factor 3) that was discussed above is used in an SFN withomni-directional antennas.

In this example, we divide the K sub-carriers into groups of size 2(i.e. L=2) so that there are K/2 sub-carrier groups in all. We assumethat sub-carriers 1 and 2 are in group 1, sub-carriers 3 and 4 are ingroup 2, and so on. (In general, for k=1, 2, . . . , K/2, the kthsub-carrier group consists of sub-carriers 2 k−1 and 2 k.) Also, eachblock consists of 2K modulated data symbols. FIG. 6 shows a possibleassignment of these data symbols to sub-carrier groups according to oneembodiment.

Once the 2K modulated data symbols in a block have been assigned to theK/2 sub-carrier groups, for each sub-carrier group the base stationcomputes the eight symbols that are to be transmitted over the next fourtime-slots. As in the previous example, the column vector of the codematrix G that has been assigned to the base station determines therelationship between the modulated data symbols assigned to asub-carrier group and the symbols that are actually transmitted overthat sub-carrier group. Once the eight symbols that are to betransmitted are computed, they can be transmitted using the eighttransmission opportunities the two sub-carriers provide over the next 4time-slots. There is no requirement that these eight symbols beingtransmitted need to be assigned to the eight transmission opportunitiesin a specific manner. However, this assignment has to consistent overall the base stations participating in the SFN. FIG. 7 shows a possibleassignment of the eight transmitted symbols to the eight transmissionopportunities provided by the kth sub-carrier group at a base stationthat has been assigned column A of the code matrix. Assignment oftransmitted symbols to transmission opportunities can be carried out ina similar manner for base stations using column B or column C of thecode matrix.

It is easy to see that similar schemes can be constructed withsub-carrier groups of different sizes (e.g., 4 or 8 in the presentcase.) In general, any sub-multiple of the code parameter “p” can beconsidered a candidate for “L”, which represents the size of asub-carrier group. What choice of L would be appropriate for a given SFNwould depend on the interplay between two factors: coherence time andcoherence bandwidth for typical users of the SFN. A large coherence timemeans that channel coefficients remain essentially unchanged for longperiods of time. Such would be the case for static users or mobile usersmoving at pedestrian speeds. If most of the users are likely to fallinto this class, there would be no need to shorten the length of timeover which one block of symbols is transmitted. That is, there may belittle motivation to use sub-carrier groups with multiple sub-carriers.(L=1 would be quite appropriate in this case.) However, in a vehicularenvironment with short coherence times, one would feel the need toshorten the time over which a block of symbols is transmitted; i.e. onewould try to use as large a sub-carrier group size as possible. Howlarge this size can be depends on the typical coherence bandwidth of thesystem, which indicates how much channel coefficients vary across thefrequency band. A large coherence bandwidth (which occurs if the delayspread at the receiver device is small) would permit the use of largevalues of L. All in all, the choice of L would be decided by the typicalcoherence bandwidth and coherence time of the intended environment.

All of the examples we have presented so far utilize rate ½ complexorthogonal codes. It should be clear, however, that the method we havepresented is not restricted to the use of rate ½ codes only. If a higherrate code can be found that fits in with the desired reuse pattern, thatcode can be used in conjunction with the proposed method. We illustratethis point using a rate ¾ code. The code matrix, G, associated with thiscode is given by:

Col. A  Col. B   Col. C $G = \begin{bmatrix}{x\; 1} & {x\; 2} & {x\; 3\text{/}\sqrt{2}} \\{{- x}\; 2*} & {x\; 1*} & {x\; 3\text{/}\sqrt{2}} \\{x\; 3*\text{/}\sqrt{2}} & {x\; 3*\text{/}\sqrt{2}} & {{\left( {{{- x}\; 1} - {x\; 1*{+ \; x}\; 2} - {x\; 2}} \right.{*)}}\text{/}2} \\{x\; 3*\text{/}\sqrt{2}} & {{- x}\; 3*\text{/}\sqrt{2}} & {{\left( {{x\; 1} - {x\; 1*{+ \; x}\; 2} + {x\; 2}} \right.{*)}}\text{/}2}\end{bmatrix}$

In order to use the space-time code associated with the above codematrix, one will have to implement a reuse pattern with the reuseparameter equal to 3. Thus, base stations (or sectors) will be labeledA, B or C, and depending upon the label associated with a base station(or sector), the base station will use the transmission pattern given bythe corresponding column of the code matrix. The stream of symbols to betransmitted is divided into groups of three symbols each, with each suchgroup transmitted over four time-slots. Thus, if x1, x2 and x3 aresymbols belonging to such a group, all base stations (or sectors)labeled A will transmit the symbols x1, −x2*, x3*/√2 and x3*/√2 over thefour time-slots allocated for the transmission of that group. Thoselabeled B will transmit the symbols x2, x1*, x3*/√2 and −x3*/√2 over thesame interval whereas those labeled C will transmit x3/√2, x3/√2,(−x1−x1*+x2−x2*)/2 and (x1−x1*+x2+x2*)/2.

It is easy to see that the above code has a coding rate of ¾ since eachantenna transmits three symbols over four time-slots. Since the symbolx3 is transmitted twice by each antenna, in order to have identicalenergy per symbol irrespective of which symbol (or combination ofsymbols) is being transmitted, it is desirable to haveE[|x₁|²]=E[|x₂|²]=E[|x₃|²]= 4/3E where E=E[|x|²] is the average transmitenergy per symbol as determined by the antenna power constraints. Bywriting out expressions for the real and imaginary parts of the receivedsymbols in terms of the real and imaginary parts of the transmittedsymbols (i.e., x1, x2 and x3), one can show that a linear processingoperation at the receiver (similar to the pre-multiplication by the Ĥmatrix that was used in the previous examples) can be used to obtainestimates of the transmitted symbols. The resulting signal-to-noiseratio is given byρ= 4/3(|h ₁|² +|h ₂|² +|h ₃|²)E[|x| ²]/σ².

While the SINR for this rate ¾ code is a little smaller than that forthe rate ½ codes, the increase in the coding rate (¾ instead of ½)yields additional bits that can be used to provide stronger channelcoding for additional protection.

While reuse patterns of 3 and 7 have been described and illustratedabove, it will be appreciated that other reuse patterns may be employed.Also, while the example embodiments were described with respect to four(in the case of the rate-¾ code) and eight time slots, it will beappreciated that more or less than eight and four time slots arepossible.

The examples described above comprised SFNs wherein all base stationshad either omni-directional antennas or they all had sectorized antennaswith the same number of sectors (coverage areas) per base station. It isnot a requirement of the present invention that this be so. The presentinvention can easily be applied to SFNs comprising base stations withdifferent numbers of sectors—for example, some with a single sector,some with three sectors, some with six sectors, and so on. When applyingthe present invention to such an SFN, the code matrix and thecorresponding reuse parameter should be selected so that column vectorsof the code matrix can be assigned to different coverage areas to ensurethat no two adjacent coverage areas are assigned the same column vector.

Another implicit assumption in the description of the examples givenabove was that there was a single transmit antenna for each sector orcoverage area associated with a base station. That is, in the case of abase station with an omni-directional antenna, the base station's onlycoverage area was served by a single antenna, whereas in the case of abase station with multiple sectors or coverage areas, each sector orcoverage area was served by a single antenna. The present invention isnot restricted to such scenarios. It is easy to apply the presentinvention to scenarios where one or more of the sectors (i.e. coverageareas) associated with one or more base stations are served by more thanone antenna. The present invention can be applied to such scenarios intwo ways:

-   -   1. When multiple antennas serve a given coverage area associated        with a base station, all of them are assigned distinct column        vectors of the code matrix. Thus, not only adjacent coverage        areas are assigned distinct column vectors of the code matrix,        but also overlapping coverage areas (i.e. those associated with        different antennas serving the same coverage areas belonging to        a base station) are assigned distinct column vectors of the code        matrix.    -   2. If multiple antennas serve a given coverage area associated        with a base station, all of them use the same column vector of        the code matrix. Thus, in this assignment, overlapping coverage        areas are assigned the same column vector of the code matrix        whereas adjacent coverage areas are assigned distinct column        vectors of the code matrix. (It is also possible to envision an        intermediate scenario where the multiple antennas serving a        given coverage area are divided into subsets, with a distinct        column vector assigned to each subset; all antennas within a        subset use the same column vector.)

In the first assignment pattern (in the case of multiple antennasserving a given coverage area), the reuse parameter (i.e. the number ofcolumns of the code matrix) is expanded to satisfy the requirement thatall adjacent as well as overlapping coverage areas be assigned differentcolumn vectors of the code matrix. For instance, recall that in thefirst example with a single antenna per coverage area, a code with reuseparameter 3 was adequate to ensure that no two adjacent coverage areaswould be assigned the same column vector of the code matrix. With twoantennas per coverage area, a code with reuse parameter 3 would not beable to meet this requirement if the two antennas serving a coveragearea are to be assigned distinct column vectors. In this case, a codewith reuse parameter 6 will be needed to ensure that no pair of adjacentor overlapping coverage areas is assigned the same column vector of thecode matrix.

In the second assignment pattern, where all antennas serving a givencoverage area are assigned the same column vector of the code matrix,the reuse parameter (and the corresponding space-time code) that wasadequate for the case of a single antenna per coverage area can beadopted to meet the requirement that no pair of adjacent coverage areasis assigned the same column vector. (In this assignment pattern,overlapping coverage areas, by definition, are assigned the same columnvector.)

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the invention, and all such modifications are intended tobe included within the scope of the invention.

We claim:
 1. A method of transmitting data in a single frequencynetwork, comprising: constructing a generalized orthogonal space timecode matrix having at least three columns, each of the columns of thematrix storing a distinct column vector therein; assigning the columnvectors to antennas serving coverage areas of a plurality of basestations such that each coverage area is associated with a different oneof the column vectors; and transmitting same data from each of theplurality of base stations such that the same data transmitted by eachof the plurality of base stations in a first coverage area of thecoverage areas is transmitted using the column vector associated withthe first coverage area and in coverage areas that are adjacent to thefirst coverage area the same data is transmitted by the plurality ofbase stations in the adjacent coverage areas using the column vectorsassociated with the adjacent coverage areas, wherein the constructingstep constructs the matrix and the assigning step assigns the columnvectors such that transmission in the coverage areas adjacent to thefirst coverage area by the plurality of base stations using a same oneof the column vectors does not occur.
 2. The method of claim 1, whereinone or more of the base stations are omni-directional such that each ofthe omni-directional base stations has a single coverage area.
 3. Themethod of claim 1, wherein one or more of the base stations aresectorized such that each of the sectorized base stations has more thanone coverage area.
 4. The method of claim 1, wherein the assigning stepassigns the column vectors based on a reuse parameter such that no twoadjacent coverage areas are assigned the same column vector.
 5. Themethod of claim 4, wherein the generalized orthogonal space time codematrix has a number of columns based on the reuse parameter.
 6. Themethod of claim 1, wherein, at each base station, the transmitting stepcomprises: dividing the data into sets of symbols; and transmitting eachset of symbols in each of the coverage areas of the base station.
 7. Themethod of claim 6, wherein the transmitting each set of symbols steptransmits each set of symbols over a plurality of time slots.
 8. Themethod of claim 7, wherein the generalized orthogonal space time codematrix has a number of rows equal to a number of the plurality of timeslots.
 9. The method of claim 6, wherein, for each of the plurality ofbases stations, the transmitting each set of symbols step transmits eachset of symbols over a plurality of time slots on a correspondingsub-carrier belonging to a set of sub-carriers associated with thesingle frequency network.
 10. The method of claim 1, further comprising:assigning a plurality of sub-carriers to each of the plurality of basestations; dividing the plurality of sub-carriers into groups; andwherein the transmitting step includes, for each base station, dividingthe data into sets of symbols; assigning the sets of symbols to thesub-carrier groups; and transmitting each set of symbols over theassigned sub-carrier group.
 11. The method of claim 10, wherein thetransmitting each set of symbols step transmits each set of symbols overone or more time slots.
 12. The method of claim 1, wherein thetransmitting step transmits the same data from at least two antennas ofat least one base station, the at least two antennas have overlappingcoverage areas, and each of the overlapping coverage areas is assigned adifferent column vector.
 13. The method of claim 1, wherein thetransmitting step transmits the same data from at least two of theantennas of at least one base station, the at least two antennas haveoverlapping coverage areas, and each of the overlapping coverage areasis assigned a same column vector.
 14. A single frequency network,comprising: a plurality of base stations each having at least onecoverage area, each coverage area associated different column vectors ofa generalized orthogonal space time code matrix, the generalizedorthogonal space time code constructed such that the matrix has at leastthree columns, each of the columns of the matrix storing a distinct oneof the column vectors therein, the base stations configured to performat least one of broadcasting and multicasting of data over a samefrequency to one or more users within the at least one coverage area,antennas associated with each coverage area being assigned the columnvectors such that each coverage area is associated with a different oneof the column vectors, the plurality of base stations transmitting thedata in each coverage area such that the data transmitted by each of theplurality of base stations in a first coverage area of the coverageareas is transmitted using the column vector associated with the firstcoverage area and in coverage areas that are adjacent to the firstcoverage area the same data is transmitted by the plurality of basestations in the adjacent coverage areas using the column vectorsassociated with the adjacent coverage areas, wherein the constructing ofthe matrix and the column vector assignment is such that the datatransmitted in the coverage areas adjacent to the first coverage area bythe plurality of base stations using a same one of the column vectorsdoes not occur.
 15. A method of receiving data in a single frequencynetwork at one or more users tuned in coverage areas of a plurality ofbase stations, each coverage area associated with a column vector of ageneralized orthogonal space time code matrix constructed to have atleast three columns, each of the columns of the matrix storing adistinct column vector therein, the column vectors assigned to antennasserving the coverage areas such that each coverage area is associatedwith a different one of the column vectors, the method comprising:deriving a modified received signal vector from received signal samples,the signal samples received at the one or more users in a first coveragearea of the coverage areas being encoded using the column vectorassociated with the first coverage area and the signal samples beingreceived at the one or more users in coverage areas that are adjacent tothe first coverage area being encoded using the column vectorsassociated with the adjacent coverage areas; multiplying the modifiedreceived signal vector by a receiver filter matrix, the receiver filtermatrix including at least one channel estimate coefficient associatedwith each of the column vectors of the generalized orthogonal space timecode matrix to obtain decision metrics; and decoding data represented bythe received signal samples based on the obtained decision metrics.